Re: btrfs-progs 4.4 re-balance of RAID6 is very slow / limited to one cpu core?

[Christian Rohmann <crohmann@netcologne.de>]

On 02/01/2016 09:52 PM, Chris Murphy wrote:
>> Would some sort of stracing or profiling of the process help to narrow
>> > down where the time is currently spent and why the balancing is only
>> > running single-threaded?
> This can't be straced. Someone a lot more knowledgeable than I am
> might figure out where all the waits are with just a sysrq + t, if it
> is a hold up in say parity computations. Otherwise perf which is a
> rabbit hole but perf top is kinda cool to watch. That might give you
> an idea where most of the cpu cycles are going if you can isolate the
> workload to just the balance. Otherwise you may end up with noisy
> data.

My balance run is now working away since 19th of January:
 "885 out of about 3492 chunks balanced (996 considered),  75% left"

So this will take several more WEEKS to finish. Is there really nothing
anyone here wants me to do or analyze to help finding the root cause of
this? I mean with this kind of performance there is no way a RAID6 can
be used in production. Not because the code is not stable or
functioning, but because regular maintenance like replacing a drive or
growing an array takes WEEKS in which another maintenance procedure
could be necessary or, much worse, another drive might have failed.

What I'm saying is: Such a slow RAID6 balance renders the redundancy
unusable because drives might fail quicker than the potential rebuild
(read "balance").



Regards

Christian